1. Field of the Invention
The present invention relates to an open type compressor, and especially relates to an open type compressor which is suitable for a steam compression type cooling cycle using a coolant in the supercritical area of carbon dioxide (CO.sub.2) and the like.
This application is based on Japanese Patent Application No. Hei 11-1661694, the content of which is incorporated herein by reference.
2. Description of the Related Art
Recently, from the point of view of protection of the environment, a cooling cycle which uses carbon dioxide (CO.sub.2) as a working gas (coolant gas) has been proposed for steam compression type cooling cycles, as a measures for elimination of fluorocarbons (refer to Japanese Patent Application, First Application No. Hei 7-18602, for example). The operation of this cooling cycle (hereinafter called CO.sub.2 cycle) is similar to the conventional steam compression type cooling cycle. That is, as shown in a line A-B-C-D-A in FIG. 6 (CO.sub.2 Moller diagram), gaseous CO.sub.2 is compressing by a compressor (A-B), this gaseous CO.sub.2 which is compressed at a high temperature is cooled by a radiator (gas cooler) (B-C), the pressure of the gas is reduced by a decompressor (C-D), the CO.sub.2 which is changed to liquid phase is evaporated (D-A), and an external fluid such as air is cooled by the a latent heat of evaporation.
However, if the external temperature is high, during the sur season or the like, the temperature of the CO.sub.2 at the radiator side becomes higher than the critical temperature of CO.sub.2, because the critical temperature of CO.sub.2 is about 31.degree. C. which is lower than that of the fluorocarbons used as conventional coolants, and therefore, CO.sub.2 does not condense at the radiator side (the line BC does not cross a saturation line SL in FIG. 6). Furthermore, the phase of CO.sub.2 at the outlet side of the radiator (point C in FIG. 6) is determined by the exhaust pressure of the compressor and the CO.sub.2 temperature at the outlet side of the radiator, and the CO.sub.2 temperature at the outlet side of the radiator is determined by the radiation capacity of the radiator and the external temperature (which cannot be controlled). Hence, the temperature of CO.sub.2 at the outlet side of the radiator is substantially uncontrollable, and the phase of the CO.sub.2 at the outlet side of the radiator is controlled by the exhaust pressure of the compressor (the pressure at the outlet side of the radiator). Consequently, if the outer temperature is high during the summer season or the like, the pressure at the outlet side of the radiator must be increased as shown in line E-F-G-H-E in FIG. 6 to secure sufficient cooling capacity (difference in enthalpy), and the operation pressure of the compressor must be increased in comparison with the conventional compressor which uses fluorocarbons.
For instance, in the case of an air conditioning unit for a vehicle, the operation pressure of a compressor using CO.sub.2 is increased to 40 kg/cm.sup.2, as opposed to that of a conventional compressor R134 using fluorocarbon, which is 3 kg/cm.sup.2. Furthermore, the stopping pressure of the compressor which using CO.sub.2 is increased to 40 kg/cm.sup.2, as opposed to that of R134, which is 15 kg/cm.sup.2. Consequently, in the case of the CO.sub.2 cycle, the differential pressure between the internal pressure of the compressor and the atmospheric pressure is increased, and therefore, there is concern of a gas leak from a shaft sealing portion of the compressor during the operation and stopping of the compressor. That is, in the conventional compressor, sufficient lubricating oil is supplied to the compressor, and this lubricating oil is partly supplied to the shaft sealing portion. However, the pressure of the lubricating oil may not be kept at a sufficiently high level, and gas leaks from the shaft sealing portion of the compressor are apt to occur. Especially, when the operation is stopped, the lubricating oil is not sufficiently supplied to the shaft sealing portion, and the gas leak fran the shaft sealing portion can easily occur. Furthermore, the shaft sealing portion may be damaged at the restart of the compressor because lubricating oil is not supplied while it is stopped. For the above reasons, the operation of the CO.sub.2 cycle is not efficient and an improvement is strongly required. Besides, Japanese Patent Application, Second Publication No. Hei 3-6350 discloses a sealing apparatus for a shaft to seal a shaft-end portion of a screw type compressor. In this apparatus, a mechanical seal and a plain bearing which acts as a labyrinth seal are separately arranged on the shaft-end portion to form an enclosed chamber between the seals. A lubricating material is sent into the chamber with a pressure which is higher than the pressure in a pump chamber, and gas leakage from the pump chamber is prevented. However, this apparatus is only for preventing the gas leakage during the operation, and is not for lubricating the machine room (pump chamber) of the compressor.
The present invention is provided in compliance with the above problems of the conventional art, and the object of the present invention is to provide an open type compressor which can secure efficient and appropriate operation during the cooling cycle by improving the lubrication ability during the operation and by preventing the leakage of the working gas when the operation is stopped.